Process for the purification of a pulp by oxidation with oxygen



y 1966 SADAYOSHI WATANABE 3, PROCESS FOR THE PURIFICATION OF A PULP BY OXIDATION WITH OXYGEN Filed Sept. 30, 1964 5 Sheets-Sheet 1 1 LL] m z D 2 [I O ..J I O 2 4 6 a 10 NaOH SOLUTION cowcm) Fig 2 I 0 U 2 .5 g

D D- I 9 E 3 LT.

I I l 2 4 6 8 l0 NGOH SOLUTION CONC lNV NTOR SADAyosHI ATANAB A'r'roRNEY y 1956 sAoAYosl-n WATANABE 3,251,730

PROCESS FOR THE PURIFICATION OF A PULP I BY OXIDATION WITH OXYGEN 'lled Sept :50, 1964 5 Sheets$heet 2 mmmEnz UZEOJIU N0 C0 SOLUTIO N CONC Nag e0 S'OLUTION CONC,(%)

INyE -OR SADAyosHI WA ATTORNEY May 17, 1966 SADAYOSHI WATANABE 3,251,730

PROCESS FOR THE PURIFICATION OF A PULP BY OXIDATION WITH OXYGEN Filed Sept. 30, 1964 5 Sheets-Sheet 5 E l 5 i5- 5: 3 c:

m 8 2 3 t u. 3 :1: V w *6 2 E I! m 2: 03 (0 3 l 2 3 4 N0 CO SOLUTION CONC.(%)

o: 5 m m 2; g 4 w 2 E w 34/ z I E 2 2 Y I o l Na OH SOLUTION CONC.(%)

INYENTOR AD AyoSHl VI/ATANAHE ATTORNEY digested pulp containing bleachable constituents.

United States Patent M 3,251,730 PROCESS FOR THE PURIFICATION OF A PULP BY OXIDATION WITH OXYGEN Sadayoshi Watanabe, 755 Higashi S-chome, Kita 26- 0, Sapporo-shi, Hokkaido, Japan Filed Sept. 30, 1964, Ser.:No. 400,496 Claims priority, application Japan, July 14, 1960,

7 Claims. (Cl. 162-56) This is a continuation-in-part of my application Serial No. 122,261, filed July 6, 1961, and now abandoned.

The present invention relates to a process for refining More specifically, the present invention relates to a process for treating such pulps with oxygen to oxidize non-cellulosic impurities in the pulp and thereby solubilize the impurities for easy removal from the treated pulp. This is accomplished in accordance with the present invention without degrading the pulp or etfecting appreciable depolymerization of the cellulosic molecules of the pulp.

Digested pulp contains such impurities as lignin, resin, and pentosanes. Prior art refining processes for removing these impurities generally comprise (l) Chlorination-40 convert lignin into alkali-soluble chlorinated lignin and,

(2) Alkali extraction.to extract alkali-soluble chlorinated lignin formed by the chlorination as well as resin and pentosanes.

In addition, according to conventional prior art processes for refining sulfite pulp for use in paper-making, three stages have been utilized. This comprises chlorination, alkali extraction and hypochlorite bleaching. It is also known that a five-stage refining process comprising third and fourth steps of chlorination and alkali extraction, as well as six-stage refining comprising two additional separate bleaching steps have been carried out on pulp used for the production of rayon. Sulfate pulp as is known, is more difficult to bleach than sulfite pulp and sometimes requires additional stages of refining. As the number of refining stages increases, the operation becomes not only more complicated but also more expensive as increased amounts of chlorine, alkali, and other chemical reagents are consumed.

It is an object of the present invention to provide a process for refining digested pulp without using chlorine to make the lignin alkali soluble.

A further object of :the instant invention is to provide a process for refining digested pulp while minimizing depolymerization of the cellulose molecules.

An additional object of the instant invention is to obtain a pulp of high whiteness and which can be readily bleached by means of oxygen or air.

Still another object of this invention is to provide a process that is more economical than prior art refining processes.

The aforementioned objects are accomplished by immersing a digested pulp substantially free from chlorine in an aqueous alkaline solution to form an impregnated wet pulp. Excess alkaline solution is :then removed by compressing the impregnated wet pulp to a critical pulp consistency. The compressed wet pulp is then disintegrated and reacted with air or oxygen under pressure at an elevated temperature.

In the alternative, the compression step may be avoided by adding a measured amount of the aqueous alkaline solution to the digested pulp so that the pulp consistency, nevertheless, is within the critical limitations.

The pulp consistency is the ratio of the weight of the dry pulp to the weight of the wet pulp. For purposes of this invention it is expressedon a percentage basis. It

3,251,730 Patented May 17, 1966 I is important that the pulp consistency be in the range between about 20% to 66.7%.

In addition to the criticality of the pulp consistency it TABLE 1 [Changes in pulp viscosity and degree of polymerization by air oxidation ailtlerltreatment with 1% caustic soda in the case of unrefined North Sea P D Air oxidation temperature( O.) I Prior to oxidation Directly oxidized:

Pulp viscosity 7.06 6.89 6.55 6.06 5.60 Degree oipolymerizatiom". 1,080 1,060 1,030 975 920 Chlorinated and oxidized:

Pulp viscosity 7.54 4.09 3.94 3.73 3.32 Degree otpolymerization 1,120 720 700 665 600 l JIS pulp viscosity (refer to Japanese Industrial Standard, page TABLE 11 [Changes in pulp viscosity and degree of polymerization by air oxidation after treatment with 1% sodium catbonate] Air oxidation temperature 0.) Prior to oxidation Directly oxidized:

Pulp viscosity 7. 06 6. 45 5. 83 Degree of polymerization 1, 080 1, 015 950 Chlorinated and oxidized:

- Pulp viscosity 7. 54 4. 67 3. 00 Degree of polymerization 1, 120 805 565 The above test data show that there is a substantial reduction in the pulp viscosity which is a measure of the molecular weight of .the cellulose molecules when the pulp is chlorinated, and that there is considerably smaller reduction in the pulp viscosity when unrefined pulp in accordance with the present invention is subjected to air oxidation. This difference depends upon .the presence or absence of chlorine in the pulp to be treated. Upon the treatment of a chlorinated pulp wherein a substantial amount of chlorine exists as a chlorinated lignin, oxygen would attack the cellulose of pulp and result in a substantial reduction of the pulp viscosity. Whereas, upon the treatment of a digested pulp substantially. free from chlorine, oxygen would hardly attack the cellulose, but oxidize the lignin to render it alkali-soluble.

A pulp as chlorinated and washed with 'water does not come within the scope of the present invention. However, the digested pulp treated by the process of this invention may be unbleached or it may also be partially bleached. Thus it is possible to have a partially bleached pulp by using, for example, a conventional chlorination and alkali extraction treatment prior to the oxidation of this invention, since the alkali extraction treatment removes chlorinated lignin. In addition it would also be possible to utilize the conventional chlorination and alkali extraction subsequent to a partial bleach by the process of this invention. Such modifications would be obvious,

FIG. 1 to FIG. 6 of the accompanying drawing show graphs of theexperimental results in examples of the process according to this invention. FIG. 1 to FIG. 6 illustrate the variations in physical and chemical properties of unbleached sulfite pulps of Japanese red pine and beech against the concentration of the solution of caustic soda or sodium carbonate. 7 1

As will be clear from Example I mentioned below,

it has been found that the air-oxidation of unbleached pulp and the cooking of it together with an additional amount of water can lead to advantageous efiects over the first stage chlorination and the second stage alkalitreatment. Thus it is found that the air-oxidation is comparable to the first stage chlorine-treatment of the prior art.

The omission of the first stage chlorine-treatment may obtain a saving of 70-80 percent in the quantity of the chlorine used in the refining process. In addition, the consumption of alkali in the process of the present invention is also minimized. For instance, when a pulp is impregnated with a 1% solution of caustic soda and then compressed to such an extent that the pulp consistency 4 tration from 0.1 to 2.0%. Furthermore, the alpha-cellulose is then rather improved.

In addition, it has been found that an approximately similar effect can be obtained when sodium carbonate is used in place of caustic soda. This will effect further economics as sodium carbonate is less expensive than caustic soda. Thus, it has been found that when sodium carbonate is used in place of caustic soda, the removal of lignin, resins and pentosanes from the sulfate pulp can proceed in much the same manner. Moreover, the processusing sodium carbonate has the further advantage, that the oxidation of the cellulose is very moderate as compared to the caustic soda process.

As will be clear from FIG. 5, when 0.5% Na CO is used in the process of the invention, the strength of paper made from the pulp is higher than that made from the raw material.

Generally, sulfate is not easily bleached. However, it has been found that the brightness of the sulfate pulp is considerably improved in accordance oxidation of this invention.

It has also been found that the mult'i-stage air-oxidation using'a diluted alkali can lead to very good results in suppressing the depolymerization of the cellulose and obtaining a higher purification effect.

The results of the analysis of a pulp treated with the two-stage air-oxidation by using 0.5 sodium carbonate are compared to the results of the analysis of the pulp when treated with the one-stage oxidation, as shown in Table III.

TAB LE III [Comparison of one-stage oxidation with twostage oxidation] Sodium carbonate Chlorine Pulp vis- Llgnin Pento- Pitch eoncen- No. cosity (percent) saues (percent) tration (percent) (percent) Raw material (Japanese red pine) 1. 43 5. 60 0. 43 9. 08 1. 24

a. One-stage oxidation 0. 5 0. 74 5. 50 0.30 5. 73 0. 36 1. 0 0.50 5. 0. 28 3. 37 0. 26 l. 5 0. 38 5. 34 0. 20 3. 93 O. 19 I 2. 0 0.35 5. 08 0. 12 3. 33 0. 16

b. Two-stage oxidation. g 36 31 0 85 22 is 33.3%, the proportion of the alkali to the pulp is only 2/100. The proportion of alkali to the pulp may be calculated from the following equation:

100P C P "To where In practice, the amount of the alkali used in the air oxidation may be directly diluted with water and utilized in the subsequent alkali-cooking stage, so that the quantity of the alkali actually used is not difierent from that used in the prior art refining processes. ,On the other hand, the quantity of chlorine utilized is significantly reduced, which is one of the remarkable features of the present invention.

It would have been expected that the oxidation of the non-cellulosic components due to the air-oxidation would have substantially depolymerized the cellulose. However, as will be clear from the graph of FIG. 2 showing the results of Example 1, the pulp product of Example 1 can be used as the pulp for use in the production of ray-on without any difficulty, because the pulp viscosity is not too reduced and amounts to about 4.5 when the concentration of the caustic soda solution has a concen- The amount of sodium carbonate required for the twostage air-oxidation is only 2% of the weight of the pulp. When comparison is made, it is found that if the onestage oxidation is carried out using the same amount of sodium carbonate, namely, in the case of 1% sodium carbonate solution, the degree of chlorine absorption is higher than in the two-stage air-oxidation and the content of lign-in is zero in the latter case. From the foregoing it. can be said that the multi-stage air-oxidation with a diluted alkali is an ideal process for the purification of pulp since it generally does not damage the fibres and gives a higher yield of pulp.

The above-mentioned air-oxidation and oxygen-oxida-- Example 1 Unbleached sulfite pulps made from Japanese red pine and from beech, respectively, are employed as the raw material. These pulps are intended for use in making rayon. Twenty grams of the pulp are immersed in aqueous solutions of caustic soda in various concentrations, ranging from 0.5% to 10% at room temperature with the oxygen-' for about-1 hour. The impregnated wet pulp is then compressed to such an extent that the pulp consistency is about 33.3%. solution being impregnated in the pulp is two times larger than the weight of the pulp itself. The wet pulp is then completely disintegrated and subsequently placed in a closed vessel having a capacity of 4 litres which contains an amount of water on its bottom. Compressed air is blown into the vessel to a pressure of 3.5 l g./cm. and the air-oxidation is carried out at 80 C. for four hours. After the oxidation has been completed,the pulp is removed, completely washed with water, and then dried.

The variation in various properties of the pulps treated Thus, the quantity of the caustic soda Example 2 This example illustrates the air-oxidation when the' TABLE IV [Refining efiect of the air-oxidation when the pulp consistency is varied] Pulp consistency 1 (raw 62.5% 47.6% 33.3% 25% 15.6% 10% material) Chlorine No l. 42 0. 95 0. 70 0.56 0. 38 0. 48 0. 53 0.65 Pitch content- 1. 24 0.30 0. 30 0. 27 0. 0. 25 O. 24 0. 18 Pulp brightness- 61. 0 64. 0 70. 5 72. 5 73. 5 74. 5 72. 0 72. 0 Pulp viscosity-.- 5. 6 5. 5 5. 2 5. 3 5. 4 5. 4

with the above experiment against the concentration of Example 3 the NaOH solution is shown in the graph of FIG..1. Curves 11 and 12 show the variations in the chlorine number of the Japanese red pine pulp and of the beech pulp, respectively. In the case of the Japanese red pine pulp, unbleached pulp shows a chlorine number of 1.43%, which falls down to about 0.4% by the air-oxidation. In the case of the beech pulp, unbleached pulp shows 'a chlorine number of 2.64%, which falls down to 0.60.7% by the air-oxidation. Thus, the initial value of the chlorine number is reduced to about 25 by the airoxidation. As to the analysis of lignin, the Japanese red pine pulp contains 0.43% lignin when it is in the unbleached condition. However, the oxidized pulp does not contain any lignin. The beech pulp contains 2.55% lignin in the unbleached condition, which is reduced to 0.35% upon the treatment with 2% caustic soda.

Curves 13 and 14 show the variations in the brightness of the Japanese red pine pulp and of thebeech pulp, re-

spectively. It is readily seen from these curves that the air-oxidation is eliective in improving the brightness of the pulp. The brightness of Japanese red pine pulp increases from 61 in the unbleached condition to a maximum of 78 after treatment with caustic soda. The brightness of beech-pulp increases from 45 in the unbleached condition to a maximum of 74 after treatment with caustic soda.

Curves 23 and 24 show a decrease in the pitch of Japanese red pine pulp and of the beech pulp, respectively, due to the air-oxidation. The pitch of the Japanese red pine pulp is reducedto about 0.2% (minimum value: 0.12%) and that of the beech pulp is reduced to about 0.4%. It is thus seen that the air-oxidation according to the present invention is very effective for the removal of resinous matters.

Curves 21 and 22 show the variations in the HS pulp viscosity of the Japanese red pine pulp and of the beech pulp, respectively. When the concentration of caustic soda is less than 21%, the de-polymerization of the cellulose is negligibly slight. In this time, however, the content of u-cellulose is not decreased. Unbleached Japanese red pine pulp contains 87.0% of a-cellulose but this content is increased to 89.1% after the treatment with 0.5% caustic soda and to 89.5% after the treatment with 1% caustic soda, respectively. The content of the pentosanes is decreased by the air-oxidation. In any case, it has been observed that in accordance with the air-oxidation the grade of the unbleached pulp is considerably improved and the caustic soda used may be at a relatively lower concentration (on the order of 2% or less) and a small ratio of the amount of alkali to the amount of pulp may 7 be effectively employed.

This example illustrates the air-oxidation of unbleached sulfite pulp which has been previously treated with an aqueous solution of sodium carbonate.

The raw material used is made of unbleached sulfite pulp of Japanese red pine which is intended for use in the production of rayon. The pulp is immersed in aqueous solutions of sodium carbonate in various concentrations at room temperature for 24 hours and then the impregnated wet pulp is compressed to a weight three times larger than the original weight of the. pulp. Thus the pulp consistency is about 33%. The pulp is thereafter subjected to the air-oxidation under the same conditions as in Example 1. FIG. 3 shows the experimental results. Curve 31 represents the variation in the chlorine number; curve 32 represents the variation in the content of lignin; curve 33 represents the variation in the content of pitch and curve 34 represents the variation inthe content of pentosanes. dium carbonate process is remarkable and comparable to that of the caustic soda process.

However, the sodium carbonate process is diflFerent from the caustic soda process in that the de-polymerization of the cellulose due to the oxidation in the pulp viscosity of the pulp so treated, against the concentration of sodium carbonate during this treatment. (As previously mentioned, the pulp viscosity is a measure of the degree ofde-polymerization.) For comparison, curve 42 represents the variation in the pulp viscosity when caustic soda ber of 0.36 and a pulp viscosity of 5.31, which is not substantiallydiflerent from 5.60. the pulp viscosity of'the raw material.

7 Example 4 Pulp treated by the air-oxidation with sodium carbonate according to the invention is treated in a beater so as to obtain a freeness of about 400 and then made into paper. Brightness and strength of the resulting paper were then determined.

FIG. 5 is a graph showing the variations in the brightness and strength of the paper against the concentration of the Na CO solution. Curve 51 represents the brightness of paper. A remarkable increase in the brightness is seen therefrom. Curve 52 represents the variation in the breaking length and curve 53 represents the variation in the burst factor of the paper., It is noted that the strength of the paper is not adversely affected when the concentration of sodium carbonate is up to 1.5%, but

The refining effect according to the sorather the strength of the paper'is improved when the concentration of sodium carbonate is 0.5%.

Example 8 subjecting it to the alkali-treatment by cooking it with alkali at 90-95 C. for four hours in a pulp concentration of 10%, the ratio of the alkali to the pulp being 6%.

Another sample of the pulp was treated'with 0.5%

caustic soda, oxidized with air and then treated with chlorine.

These samples are subsequently refined by using 6% of the alkali based on the pulp,.and the resulting semibleached pulps are further bleached. These bleached pulps are then analyzed and the results obtained are tabulated in Table V.

TABLE V [Purification conditions air-oxidation of sulfite pulp of Japanese red pine for rayon application] Air-oxidation with Air-oxidation with 1% Two-stage oxidation 0.5% caustic soda folcaustic soda with 0.5% caustic soda lowed by chlorine treatment Treatment Main conditions Semi- Semi- Semi- I bleaching Bleaching bleaching Bleaching bleaching Bleaching (1) 1st stage air-oxidation. Concentration of immer- 1.0 0.5 0.5

sion caustic soda (percent). (2) Hot washing Amount of alkali based 2.0 1.0 1.0

on pulp (percent) (no alkali added). (3) 2nd stage air-oxida Concentration of immer- 0.5

tion. sion caustic soda (percent). (4) Hot washing Amount of alkali based 1.0

on pulp (percent) (no added) (5) Chlorinetreatment.-- Amount of chlorine 1 0,35

' (based on pulp). (6) Alkali-treatment--. Amount of alkali added 6.0 6.0 6.0

(percent) (based on P D (7) Hypochlorite-treat- Available amount of Not done. Done Not done Done Not done Done ment. chlorine is 1%. (8) Acid-treatment SO; water PH 4 Not done Done Not done Done Not done Done Analytic data a-celiulose (percent)- 93. 4 92. 4 93. 6 92. 7 93. 2 93. 1 fl-cellulose (percent) 2. 63 2. 85 3. 06 3. 14 2. 82 3. 38 HS pulp V Pn y- 5. 9 4. 8 5. 7 4. 5 6. 3 5. 1 Brightness 72. 3 88. 0 73.8 i 90.0 73. 4 90. 5 Chlorine N0- 0. 25 0. 08 0.23 0. 06 0.17 0.0 Pitch content (percent) 0.12 0. 11 0. 09 0. 09 0. 10 0. 1 Lignin content (percent) 0. 0 0. 16 0. 0 O. 0 Pentosans content (percent)- 5. 2 4. 4 3. 9

1 Corresponding to 100% of chlorine absorption degree at 0 C.

tion. The brightness of the resulting pulp was considerae bly improved.

For instance, unbleached pulp obtained from the sulfate process is immersed in the sulfate digestion liquor, subsequently compressed to a consistency of about 33% and then subjected to the air-oxidation under the same conditions as in Example 1, at temperatures of C., C., and 80 C., respectively.

The brightness of the raw material pulp was only 24 but it was increased to 37 when treated at 60C.; to 48 when treated at 70 C.; and to 49 when treated at 80 C. The lignin content of the raw material pulp was 8.55% but this content of the pulp when treated at 80 C. is reduced only 1.92%, and it-is thus noted that 78% of the lignin content of the raw pulp had been removed.

Example 6 This example illustrates the production of a pulp for use in the manufacture of rayon from the air-oxidized pulp.

Unbleached sulfite pulp. of Japanese red pine is used as the raw material. A sample of-the pulp is taken and subjected to the refining process, the first step of which comprises treating the pulp with 1% caustic soda and airoxidizing it, the second step of which comprises adding an amount of water to result in a 10% pulp concentration, and subjecting the mixture to cooking at 80-85 F C. for two hours, and the third step of which comprises The results shown in Table V clearly prove that the process of the invention is suitable for refining a pulp which is intended for use in making artificial fibres and that the conventional chlorine-treatment may be completely replaced by the air-oxidation according to our invention.

Furthermore, in accordance with the invention it is also possible to yield a pulp for rayon application by allowing the cellulose to be oxidized slightly by the 'airoxidation. Since the bleaching'step may be omitted thereby, the cost of the purification is further reduced.

I Example 7 This example illustrates the oxidation with oxygen of unbleached sulfate pulp which has been previously treated with an aqueous solution of caustic soda.

FIG. 6 is a graph which shows the eifects of refining on sulfate pulps of beech and white fir which have been treated by immersing in solutions of caustic soda in various concentrations for 24 hours, compressing the impregnated pulps to a pulp consistency of about 33% and oxidizing it with oxygen under a pressure of 10 kg./cm. at C. for four hours. Curves 63 and 64 represent the improvement in the whiteness of the pulp of white fir and of the pulp of beech, respectively. Curves 61 and 62 represent the variations in the chlorine number of the same samples, respectively.

It is noted that pressures above 10 kg./cm. do not significantly improve the properties and that generally a pressure of 10 kgJcm. will suflice.

What is claimed is:

1. A process for refining a digested pulp substantially free from chlorine which comprises immersing said pulp in an aqueous medium selected from the group consisting of sodium hydroxide and sodium carbonate; said sodium hydroxide consisting essentially of a 0.1 to 2.0% solution and said sodium carbonate consisting essentially of a 0.5 to 4.0% solution; compressing the pulp to remove any excess aqueous medium so as to provide a wet mass of pulp having a pulp consistency of about 20% to about 66.7% loosening the wet mass to provide a porous pulp mass; and treating the porous pulp mass with an oxygen containing gas at a temperature of from 60 C. to 100 C.

2. The process according to claim 1 wherein an we cess of the aqueous medium is used.

3. The process according to claim 1 wherein said pulp immersed in said aqueous medium has previously been subjected to a sulfite digestion.

4. The process according to claim 1 wherein said pulp immersed in said aqueous medium has previously been subjected to a sulfate digestion.

5. The process according to claim 1 where preliminary to compressing the pulp, it is allowed to stand in said aqueous .medium at ambient temperatures for a time period in the range of from about one to about 24 hours.

6. The process according to claim 1 wherein said aqueous medium is sodium hydroxide.

7. The process according to claim 1 wherein said aqueous medium is sodium carbonate.

References Cited by the Examiner UNITED STATES PATENTS 1,860,432 5/1932 Richter 16265 2,147,618 2/ 1939 Rawling 162--65 3,024,158 3/1962 Grangaard "162-65 DONALL H. SYLVESTER, Primary Examiner. H. R. CAINE, Assistant Examiner. 

1. A PRECESS FOR REFING A DIGESTED PULP SUBSTANTIALLY FREE FROM CHLORINE WHICH COMPRISES IMMERSING SAID PULP IN AN AQUEOUS MEDIUM SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE AND SODIUM CARBONATE: SAID SODIUM HYDROXIDE CONSISTING ESSENTIALLY OF A 0.1 TO 2.0% SOLUTION AND SAID SODIUM CARBONATE CONSISTING ESSENTIALLY OF A 0.5 TO 4.0% SOLUTION: COMPRESSING THE PULP TO REMOVE ANY EXCESS AQUEOUS MEDIUM SO AS TO PROVIDE A WET MASS OF PULP HAVING A PULP CONSISTENCY OF ABOUT 20% TO ABOUT 66.7%; LOOSENING THE WET MASS TO PROVIDE A POROUS PULP MASS; AND TREATING THE POROUS PULP MASS WITH AN OXYGEN CONTAINING GAS AT A TEMPERATURE OF FROM 60*C. TO 100*C. 